Variance

Variance is a measure of the data's spread around the mean; the average of the squared distances of each value from the mean, which is also the mathematical basis of standard deviation.

Variance and standard deviation measure the same thing but in two units: variance is in the data's squared unit (like 'seconds squared'), which is hard to interpret, so reports usually state its square root, the standard deviation. So why does variance matter at all? Because in statistics it has an additive property, and deeper computations — from statistical power to analysis of variance between groups — are built on it. The conceptual point is that high variance means volatile data, and high volatility means you need a larger sample to reach a meaningful result; so the data's spread directly raises an experiment's cost and duration. Variance should not be confused with effect size; variance is about scatter, not about the difference between means.